A polyamide resin has an excellent mechanical performance and hence is widely used as a material for injection-molded products such as automobile and electrical and electronic parts. Further, the polyamide resin is used as a packaging material for foods, drinks, drugs, electronic parts, and the like. Of those, a polyamide (MX nylon) obtained by a polycondensation reaction between a xylylenediamine and an aliphatic dicarboxylic acid, in particular, a polyamide (polyamide MXD6) obtained from m-xylylenediamine and adipic acid has low permeability to a gaseous substance such as oxygen or carbon dioxide, and hence is used as a gas-barrier material in a molded product such as a film or a bottle.
In recent years, as a container for storing a fuel, a resin container for a fuel formed by direct blow molding or the like has attracted attention from the viewpoints of, for example, reducing the weight, requiring no antirust treatment, improving a degree of shape freedom, reducing the number of steps for processing, and automatizing production, and the container has been gradually substituted for a metal container for a fuel. However, a polyolefin (in particular, high-density polyethylene) used in the resin container for a fuel is excellent in mechanical strength, molding processability, and economic efficiency but has poor barrier performance against a fuel (hereinafter, also referred to as “fuel barrier property”), and hence is difficult to meet the regulation of a fuel permeation amount.
Therefore, there have been proposed, for example, a method involving performing a fluorine treatment inside a container and a multilayer container including a fuel barrier layer, which is formed of a polyamide resin, an ethylene-vinyl alcohol copolymer, or the like having fuel barrier property, as an intermediate layer between polyethylene layers (see Patent Documents 1 to 5, for example). Of those, the fluorine treatment is now less used because it is difficult to ensure safety in handling of a hazardous gas and to collect the gas after the treatment, for example. In the case of the multilayer container, the fuel permeation amount can be decreased to a certain degree by providing the fuel barrier layer, but the barrier property against a fuel is still imperfect. When the thickness of the fuel barrier layer is increased, problems such as a decrease in impact absorption at the time of impact, an increase in weight, and an increase in cost may be caused, and hence it has been difficult for the multilayer container to sufficiently meet regulations to be more tightened in the future.
Moreover, addition of ethanol or the like to gasoline can decrease the amount of a fossil fuel to be used, resulting in reducing the amount of carbon dioxide emissions. Therefore, a study on use of ethanol as a fuel has been made, but nylon 6 and an ethylene-vinyl alcohol copolymer is inferior in barrier property against an alcohol. Accordingly, a material having enhanced barrier property against a fuel containing an alcohol has been required.
Meanwhile, a container for applications such as a fuel is usually molded by a direct blowing method, but the above-mentioned conventional fuel barrier material is inferior in heat resistance and tends to cause gelation by an increase in the temperature of a resin or accumulation of a resin, which restricts conditions in production. In addition, it is necessary to recycle burrs generated in molding and to switch the resin to a resin excellent in heat stability in operating or stopping an apparatus, and there are problems in productivity and efficient use of the material. In particular, if the resin has a low melt viscosity, the direct blowing method causes drawdown to excessively decrease the thickness of the resultant product or defects such as uneven thickness. Further, when the molding temperature is too high, the melt viscosity of a polyolefin used in the outer layer is lowered to cause drawdown, which is not preferred.
In a multilayer fuel container obtained by molding a material including a polyamide resin having fuel barrier property as an intermediate layer by the direct blowing method, there is a problem in that a fuel barrier material is not fed in a pinch-off site generated during molding, and a fuel permeates from the pinch-off site, resulting in lowering the fuel barrier property. Therefore, recently, a multilayer fuel container including a polyamide resin in an inner layer and a fuel container formed of a blend of a polyamide resin with a polyolefin have been developed. In such fuel containers, the polyamide resin having fuel barrier property is contact with a fuel, and hence the fuel barrier property can be maintained to a high level.
In addition, Patent Document 6 describes a polyamide resin obtained by copolymerization of m-xylylenediamine with an aliphatic dicarboxylic acid and a naphthalenedicarboxylic acid, and describes that the polyamide resin is excellent in fuel barrier property.
Further, Patent Document 7 describes that a polyamide resin excellent in fuel barrier property, in particular, alcohol barrier property can be obtained by maintaining a polymer obtained by copolymerization of m-xylylenediamine with an aliphatic dicarboxylic acid and isophthalic acid in the presence of 1 to 30 wt % of water at 70 to 120° C. for 0.5 to 4 hours to crystallize the polymer and performing a heat treatment in an inert gas atmosphere or the like at a temperature of (the melting point of the polymer −50° C.) to (the melting point of the polymer −10° C.) for 1 to 12 hours.